Many silos which were built before the dynamics of discharge were fully understood, were designed for static loading, but it has been shown that material pressures exerted on the cell walls increases by a factor of up to about two and a half when the outloading valve is opened and material begins to move.
The opening of the valve removes vertical support from the material directly above it and the stress field changes from "peaked", with lines of major stress vertical, or near vertical, to "arched" with lines of major stress arching across the cell.
The "arched" stress field occupies a conical zone which diverges upwardly. At the point where this conical zone intersects the cell walls, the large lateral component of force causes a high hoop stress in the cell walls. This stress exceeds the static stress by a factor of up to two and a half, and is often large enough to cause overstressing and cracking of cell walls.
The cost of replacement of a silo is prohibitive, and the main object of this invention is to provide improvements whereby the hoop stress can be substantially reduced.
Several methods are available to strengthen the cylindrical walls of an upstanding silo. One widely used (but basically unsound) method, is the repair of bulged areas, but even this is expensive. The second alternative is the use of external strapping on the external surfaces of external cells only of a group of silos, but this is many times more expensive than the cost of local bulge repairs. Another possibility which has been examined has been the use of a steel liner spirally wound within a silo to lie against the inner surface of a concrete wall but this is even more expensive than the external strapping. The other alternative (apart from this invention) is the use of a concrete liner constructed for the full height of the cell and within an old cell, but the cost of this is so great that it is not viable.
The object of this invention is to provide improvements which are economical and feasible.